The ground operation of military aircraft is very similar to the operation of civilian commercial aircraft. The term “military aircraft,” as used herein, is intended to encompass and include any type of aircraft used in military operations or missions capable of travel on a ground or other surface, including, but not limited to, fixed wing jet and propeller driven aircraft, rotorcraft, such as helicopters and the like, as well as unmanned aerial vehicles (UAVs). There are significant differences that make the operation of military aircraft unique. Military aircraft must be able to land and travel on a wide range of surface types, for example, from sand and dirt to the decks of aircraft carriers. Unlike civilian aircraft, they must often be able to land and take off quietly and quickly. In certain situations, multiple military aircraft must be able to roll in and roll out simultaneously, sometimes in relatively constrained spaces. At the present time, military aircraft, like commercial aircraft, must operate the aircraft main engines to move the aircraft on tarmac or any other ground travel surface between landing and takeoff. The use of thrust from an aircraft's engines to move an aircraft during ground surface travel presents many challenges to the efficient and safe operation of all aircraft and to military aircraft in particular.
The environment surrounding an operating aircraft engine is an acknowledged hazard zone because of the dangers to ground personnel and equipment and to the aircraft itself that can accompany jet blast and engine ingestion. Engine ingestion, also referred to as jet intake, can damage engines and other aircraft structures when foreign object debris (FOD) is pulled into an operating aircraft engine, which causes foreign object damage to the engine, potentially compromising the safety of aircraft engine operation. The safety of ground operations can also be significantly compromised by the jet blast from an aircraft jet engine when aircraft engines are kept in operation, even at idle speeds, especially in tight or congested areas with reduced maneuvering space. Jet blast, also known as jet efflux, from any type of engine operating to taxi an aircraft in a congested area virtually guarantees that something will be damaged or someone will be injured. Air currents caused by prop wash from aircraft equipped with propellers or the main and tail rotors of helicopters can also present ground safety challenges. All of the foregoing risks may also be presented when an unmanned aerial vehicle (UAV) type of aircraft taxis with operating engines.
Jet blast data, measured from an aircraft's tail with the engines at low RPM settings, indicates that the damage profile can extend from outboard wing-mounted engines to more than 200 feet beyond some larger aircraft. Within this area, jet engines can generate hurricane-level exhaust forces of almost 100 knots. Most of the reported jet blast damage incidents typically occur during pushback, power back, taxi-out, or taxi-in. The position of the operating jet engines relative to ground equipment, people, and other aircraft can significantly influence the occurrence of jet blast damage incidents when breakaway power is applied. Aircraft with engines powered and in the process of turning are frequently involved in jet blast damage incidents. Using powered engines to maneuver an aircraft without assistance from a tractor or tug is highly likely to compromise ground safety. The presence of a tractor or tug, however, is not likely to prevent jet blast damage if the aircraft's engines are running and the aircraft is in the process of making a sharp turn. Careful management of an engine-powered military aircraft when the aircraft moves on the ground is required to prevent damage from jet blast or propeller operation, particularly in congested areas.
Positioning a jet or other type of military aircraft so that the forward thrust is directed away from people and equipment is helpful, but the direction of the jet blast can change as the aircraft is maneuvered on the ground or other surface. This occurs, for example, during power back operations, when the flight crew employs engine thrust reversers to direct thrust ahead of the aircraft to push the aircraft backward, changing the direction of the jet blast. Damage to other aircraft and/or to ground vehicles or ground personnel remains a distinct possibility as long as an aircraft's engines are running. Suggestions for preventing jet blast damage thus far have been limited to, for example, avoiding sharp turns on taxi-in or pushback with one or more engines running, and using tractors or tugs to move taxiing aircraft. The use of jet blast deflectors and improving ground crew vigilance, communication, and the handling of ground vehicles may reduce damage. As long as engines are operating while aircraft are on the ground, however, jet blast continues to be a hazard. Propeller driven military aircraft present their own safety challenges, and, while different, prop wash produced by operating engines and propellers still poses hazards to vehicles and personnel in the vicinity of the aircraft.
As noted above, operating military aircraft engines while the aircraft is on the ground may result in engine ingestion. The operation of an aircraft engine, whether it is a jet engine or a gas or turbine engine with an attached propeller, creates a low pressure area in the engine inlet, which causes a large quantity of air from the area forward of the inlet cowl to move into the engine. The velocity of the air nearest the inlet is much greater than the velocity of the air farther from the inlet. As a result, the amount of engine suction close to the inlet is significant and may be high enough to pull tools, equipment, and even people into the engine. To avoid the possibility of serious injury or, in rare cases, death, it is necessary for ground personnel and ground vehicles to keep a safe distance from an operating aircraft engine. The hazard or danger zone around one type of aircraft with an engine operating at idle power extends for a radius of about 9 feet (2.7 m) from the center of the engine and about 4 feet (1.2 m) back toward the engine cowl. This hazard zone increases to a radius of about 13 feet (4 m) and a distance toward the cowl of about 5 feet (1.5 m) when the aircraft engine is operating just above idle power. At higher power levels, the hazard zone increases to at least 100 feet (30.5 m) in front of the engines and at least 200 feet (61 m) behind the engines. The extent of the engine ingestion hazard zone may be increased by wind or weather conditions. Where the engine ingestion hazard zone ends in the vicinity of the engine cowl, the exhaust hazard area begins, and damage or injury from jet blast hazard is also possible. The dangers associated with operating aircraft turbines, whether they are pure jet engines, turboprop engines, or helicopter rotors, cannot be overstated. Even after an aircraft engine is shut off completely, the possibility of engine ingestion may exist for a period of about 30 seconds. Military aircraft often must operate in locations where the area of clearance around an operating engine required to avoid engine ingestion is not available.
Foreign object damage to aircraft engines from foreign object debris (FOD) picked up from adjacent ground areas by engine ingestion is a major cause of reduced engine life. Under-wing engines literally vacuum FOD from the ground, causing engine damage that can ground aircraft for expensive and time-consuming engine overhauls. Since many military aircraft must land on surfaces that are rough and may be covered with or composed of dirt, rocks, and/or sand, the ingestion of FOD from these surfaces can pose significant challenges to the continued effective operation of the aircraft engines, to the aircraft, and, ultimately, to the mission.
An aircraft can be moved in reverse from a parked position by starting the aircraft's main engines and reversing them to drive the aircraft in a reverse direction during push back or at other times. However, this process, known as reverse thrust, is problematic and can be dangerous. An aircraft engine operating in reverse thrust pulls FOD from the aircraft's environment into the engine and throws it forward. The potential for injury from FOD to ground personnel, ground vehicles, and airport or other structures where military aircraft are operating during this process can be significant. The use of reverse thrust is prohibited in many locations, moreover. Dependence on the use of an arriving or departing aircraft's main engines is neither a safe nor a reliable procedure.
In addition to the turbulence and noise created by an aircraft's engines operating in reverse thrust, as well as in idle thrust or taxi thrust, the adverse impact on air quality and fuel costs must be considered. It has been estimated that about 3200 pounds of fuel is used in an hour by an idling aircraft engine. An aircraft's engines idling between push back and takeoff, even if only about 20 minutes a day, can increase fuel costs by millions of dollars over a fleet.
In some situations, it is necessary for multiple military aircraft to conduct both push back and landing virtually simultaneously. Currently, adjacent and simultaneous stand operations are often limited significantly when surface space for multiple aircraft is tight. Moreover, jet blast deflectors are required since these multiple aircraft are simultaneously rolling in or out with their engines operating.
If tugs are used, this strains not only the available towing equipment, but also the available ground personnel. When military aircraft require tugs for push back, departures may be delayed and turnaround times can be adversely affected when tow bars, adapters, tugs, or ground crews are not available when needed. In some locations where military missions are conducted, tugs are simply not available, and aircraft must move in reverse as described above. If an aircraft is damaged during roll in or roll out in a tight space or causes damage to another aircraft when multiple aircraft must simultaneously deploy from this space, and the damage is not detected prior to takeoff because ground crew were busy elsewhere, aircraft safety and the aircraft's mission could be compromised.
The ground movement of an aircraft without the operation of the aircraft's engines has been proposed. U.S. Pat. No. 2,430,163 to Dever; U.S. Pat. No. 3,977,631 to Jenny; U.S. Pat. No. 7,226,018 to Sullivan; and U.S. Pat. No. 7,445,178 to McCoskey et al, for example, describe various drive means and motors intended to drive aircraft during ground operations. None of the foregoing patents, however, suggests a surface travel system specifically designed to move military aircraft quietly, efficiently, and safely on a wide range of travel surfaces. U.S. Pat. No. 7,469,858 to Edelson; U.S. Pat. No. 7,891,609 to Cox; U.S. Pat. No. 7,975,960 to Cox; and U.S. Pat. No. 8,109,463 to Cox et al, owned in common with the present invention, describe aircraft drive systems that use electric drive motors to power aircraft wheels and move an aircraft on the ground without reliance on aircraft main engines or external vehicles. While these drive systems and the various drive system features disclosed can effectively move aircraft on the ground, it is not suggested that they can move aircraft as required in military applications.
A need exists, therefore, for a surface travel system specifically designed to move military aircraft autonomously on a wide range of surfaces and surface conditions efficiently, quietly, and safely without the hazards that accompany operating aircraft engines in a manner that promotes the effectiveness and success of the aircraft's mission.